A conventional television receiver has a 4:3 image aspect ratio (the ratio of the width of the image to its height). Recently there has been interest in using wider image aspect ratios for television systems, such as 2:1 or 5:3, which more nearly approximate or equal the image aspect ratio of the human eye than does the conventional 4:3 aspect ratio. The 5:3 aspect ratio has received particular attention since release prints for motion picture film that use used this ratio can be transmitted and received in a 5:3 aspect ratio television system without cropping. However, wide-screen television systems which simply transmit signals having an increased aspect ratio as compared with conventional television systems are incompatible with conventional aspect ratio receivers. (As used herein, the term wide-screen means any image aspect ratio greater than the 4:3 aspect ratio used in conventional television displays.) It has been recognized by Meise et al. in their U.S. Pat. No. 4,551,754 entitled COMPATIBLE WIDE-SCREEN COLOR TELEVISION SYSTEM issued Nov. 5, 1985, that a wide-screen television signal may be made compatible with conventional television receivers by compressing or "squeezing" the left and right edges of the wide-screen image. When displayed on a conventional television receiver, the squeezed edges of the image are largely hidden from view due to receiver overscan. When displayed on a wide-screen receiver, the compressed edges are restored to their original width by means of time-expansion circuits included in the receiver.
In the Meise et al. system, picture edge squeezing is provided by modifying the horizontal deflection signal applied to the image pickup tube of the camera. In particular, additional horizontal rate deflection ramp generator circuits are used to modify the main horizontal rate deflection ramp generator output signal, so as to cause its ramp rate to increase in the vicinity of the edge portions of each horizontal line scan. This causes the image pickup tube to be scanned more quickly in the edge portions and results in readout from the pickup tube of a video signal representative of an image which is time-compressed at its edge portions. Meise et al. disclose image restoration ("de-squeezing") by means of a memory which stores the edge-squeezed video signal in response to a constant frequency write clock and recovers the stored signal in response to a variable frequency read clock. Changing the read clock frequency alters the relative timing of picture elements within the horizontal line, thereby facilitating expansion of the compressed edges of the displayed image.
To provide for display of both wide-screen and standard aspect ratio (i.e., 4:3) images, Meise et al. disclose a dual-mode receiver. In the Meise et al. system a coded signal is added to the vertical blanking interval of the compatible (edge-squeezed) wide-screen signal for identifying the signal as being representative of a wide-screen image. The coded signal is detected in the dual-mode receiver and used to control the display raster width and the time-expansion circuits. When the code is present, the time-expansion circuits are enabled and the raster width is expanded to the full width of a wide-screen kinescope. When standard television signals are received, the absence of the code is detected and used to reduce the raster width to provide a 4:3 aspect ratio and the time-expansion circuits are disabled (by-passed).
K. H. Powers in his U.S. patent application Ser. No. 504,374 filed June 14, 1983 as a continuation-in-part of application Ser. No. 485,446 filed April 14, 1983 (now U.S. Pat. No. 4,605,952, which issued Aug. 21, 1986), describes another example of a compatible wide-screen television system. In the Powers system, the center portion of the image is slightly compressed and the compression of the edges of the image ramps linearly to a factor of about 3:1 at the extreme edges. The camera in the Powers system includes CCD imagers which have an imaging area with a 5:3 aspect ratio. A wide-screen video signal is generated by clocking the imagers with clocking signals of uniform rate. Edge compression is provided by variable clock rate sampling of the wide-screen analog video signal provided by the CCD imagers. The sampling rate is varied by applying the output of a high frequency oscillator to a programmable divider having divisor coefficients stored in a programmable read only memory (ROM). The ROM is addressed by a counter that is clocked during each line interval, thereby changing the divisor coefficents and changing the sampling frequency during each line interval. This results in edge compression of the wide-screen video signal and generation of a wide-screen signal which is compatible with conventional aspect ratio television receivers.
It is recognized by the present inventor that a need exists for a compatible wide-screen television signal generator which does not require the use of auxiliary circuitry, such as additional ramp generators for modifying the pick-up tube deflection signals or high frequency and variable frequency oscillators, for providing edge compression of a wide-screen signal.